1. Field of the Invention
The present invention relates to circuits for power conversion. More specifically, the present invention relates to control circuits for bridgeless boost power converters with power factor correction (PFC).
2. Description of the Related Art
A conventional PFC converter includes a diode bridge that provides full-wave rectification of an alternating-current input voltage such that the power-conversion circuitry of the conventional PFC converter always receives a positive-polarity input voltage. The power-conversion circuitry typically includes a boost inductor with an auxiliary winding that is used for demagnetization sensing. The auxiliary winding is coupled to a detection circuit that includes an analog comparator or an integrated circuit (IC) device.
Commonly used operation modes for a conventional boost converter include a continuous-conduction mode and a boundary-conduction mode. In the continuous-conduction mode, the current through a boost inductor of the boost converter is continuous. In the boundary-conduction mode, a switch included in the boost converter turns on when the current through the boost inductor reaches zero, which is a boundary between the continuous-conduction mode and a discontinuous-conduction mode. In the discontinuous-conduction mode, the current through the boost inductor reaches and remains at zero for a winding reset time period, which results in a “dead zone.” Accordingly, at the critical point when the current through the boost inductor reaches zero, a boost converter operating in the boundary-condition mode turns on the switch to avoid the “dead zone.” A boost converter that operates in the boundary-conduction mode eliminates the need for a fast-recovery diode that is required by a boost converter that operates in the continuous-conduction mode. Further, the switch of the boost converter is able to turn on with zero or substantially zero current in the boundary-conduction mode, which reduces switching loss in the boost converter.
A bridgeless PFC converter relies upon diodes and switches included within the switching circuitry of the bridgeless PFC converter to rectify an alternating-current input voltage. Because bridgeless PFC converters cause the input voltage to the power conversion circuitry of the PFC converter to alternate between positive and negative polarity every half-cycle of the alternating-current input voltage, conventional demagnetization-sensing circuits are generally unusable with bridgeless PFC converters. More specifically, conventional demagnetization-sensing circuits use diodes to couple the auxiliary winding of the boost inductor to the sensing circuit so that the demagnetization pulses are sent to the sensing circuit, and this diode coupling only permits a single polarity to be detected by the sensing circuit.
Accordingly, a demagnetization-sensing for a bridgeless PFC converter requires special design considerations so that demagnetization pulses of both positive and negative polarity are able to be detected. FIG. 1 shows a conventional bridgeless PFC converter 10 with a demagnetization-sensing circuit.
As shown in FIG. 1, the bridgeless PFC converter 10 includes an alternating-current power supply AC that is connected in series with a boost inductor L1. The bridgeless PFC converter 10 also includes a first transistor Q1 and a second transistor Q2 that are connected in series with each other in a totem-pole configuration, as well as a first diode D1 and a second diode D2 that are connected in series with each other. The first and second transistors Q1 and Q2 are connected in parallel with a capacitor C1. The capacitor C1 is an energy-storage bulk capacitor, and the voltage at capacitor C1 provides the direct-current output DC of the bridgeless PFC converter 10. The bridgeless PFC converter 10 may also include current-sense transformers CS1 and CS2 in series with the first and second transistors Q1 and Q2 to detect the current flowing through the first and second transistors Q1 and Q2.
The boost inductor L1 and the alternating-current power supply AC are, respectively, connected to a point between the first and second transistors Q1 and Q2 and a point between the first and second diodes D1 and D2. This arrangement allows the PFC converter 10 to operate as a bridgeless PFC converter, since the alternating-current power supply AC is rectified by the first and second transistors Q1 and Q2 and the first and second diodes D1 and D2 so that a positive voltage is always applied to the capacitor C1 at the direct-current output DC. Particularly, when the alternating-current power supply AC outputs a positive voltage, the second transistor Q2 and second diode D2 conduct to provide a charging current to the boost inductor L1, and the first transistor Q1 and the second diode D2 conduct to provide a discharging current from the boost inductor L1 to the capacitor C1. However, when the alternating-current power supply AC outputs a negative voltage, the first transistor Q1 and first diode D1 conduct to provide a charging current to the boost inductor L1, and the second transistor Q2 and the first diode D1 conduct to provide a discharging current from the boost inductor L1 to the capacitor C1. During the half-cycle when the alternating-current power supply AC outputs a negative voltage, the negative voltage of the alternating-current power supply AC is transformed to a positive voltage and supplied to the direct-current output DC.
To perform boost conversion in the PFC converter 10, it is necessary to properly control the on-off operation of the first and second transistors Q1 and Q2 so that the PFC converter 10 operates in a boundary-conduction mode. Particularly, the first and second transistors Q1 and Q2 are controlled based upon the on-time determined by a control device 11 and the magnetization state of the boost inductor L1 to obtain the desired boundary-conduction-mode operation and to obtain the desired output voltage and input current characteristics.
As shown in FIG. 1, the demagnetization-sensing circuit of the bridgeless PFC converter 10 includes a sense resistor R1, a differential amplifier 12, an isolation device ISO, and the control device 11. The sense resistor R1 is arranged in series between the alternating-current power supply AC and the boost inductor L1. A voltage drop across the sense resistor R1 is detected by the differential amplifier 12, and a signal output from the differential amplifier 12 is received by the control device 11 via the isolation device ISO. As shown in FIG. 1, the isolation device ISO may be a transformer. According to the voltage across the sense resistor R1, the control device 11 is able to detect when the boost inductor L1 becomes demagnetized by determining a zero crossing of the current in the boost inductor L1. More specifically, the control device 11 is able to detect when the current flowing through the sense resistor R1 changes from a positive current to a negative current, or when the current flowing through the sense resistor R1 changes from a negative current to a positive current.
A magnetization state of the boost inductor L1 changes according to the current flowing through the boost inductor L1. When a boost switch of the PFC converter 10 is turned on, current flows through the boost inductor L1 from the alternating-current power supply AC, generating a magnetic field and magnetizing the boost inductor L1. After the boost inductor L1 is magnetized, the boost switch is turned off and the freewheel switch is turned on to connect the boost inductor L1 to the direct-current output DC. Then, the freewheel switch is turned off, and the boost inductor L1 demagnetizes. Based upon the magnetization state of the boost inductor L1, the control device 11 outputs signals that control the on-off operation of the first and second transistors Q1 and Q2 to maintain boundary-conduction-mode operation of the PFC converter 10. More specifically, the control device 11 determines a time delay after which a boost switch is turned on to initiate a switching half-cycle. The time delay may either be computed by the control device 11, or the control device 11 may determine the time delay from a lookup table based on information including, for example, input voltage, output voltage, and load conditions.
The boost switch changes between the first transistor Q1 and the second transistor Q2 every half-cycle of the alternating-current power supply AC. That is, during the half-cycle when the alternating-current power supply AC outputs a positive voltage, the second transistor Q2 operates as the boost switch and the first transistor Q1 operates as a freewheel switch according to the on-off control signals output by the control device 11. Conversely, during the half-cycle when the alternating-current power supply AC outputs a negative voltage, the first transistor Q1 operates as the boost switch and the second transistor Q2 operates as the freewheel switch according to the on-off control signals output by the control device 11.
However, since the magnetization state of the boost inductor L1 is detected according to the voltage across the sense resistor R1, the demagnetization-sensing circuit used with the bridgeless PFC converter 10 shown in FIG. 1 reduces the efficiency of the bridgeless PFC converter 10. Specifically, the sense resistor R1 is a dissipative element that renders the demagnetization-sensing circuit unusable in high-efficiency applications or in bridgeless PFC converters that operate at high voltages.